Plasma discharges can be used to excite gases to produce activated gases containing ions, free radicals, atoms and molecules. Activated gases are used for numerous industrial and scientific applications, including processing solid materials such as semiconductor wafers, powders, and other gases. The parameters of the plasma and the conditions of the exposure of the plasma to the material being processed vary widely depending on the application.
For example, some applications require the use of ions with low kinetic energy (i.e., a few electron volts) because the material being processed is sensitive to damage. Other applications, such as anisotropic etching or planarized dielectric deposition, require the use of ions with high kinetic energy. Still other applications, such as reactive ion beam etching, require precise control of the ion energy.
Some applications require direct exposure of the material being processed to a high density plasma. One such application is generating ion-activated chemical reactions. Other applications include etching of and depositing material into high aspect ratio structures. Other applications require shielding the material being processed from the plasma because the material is sensitive to damage caused by ions or because the process has high selectivity requirements.
Plasmas can be generated in various ways including direct current (DC) discharge, radio frequency (RF) discharge, and microwave discharge. DC discharges are achieved by applying a potential between two electrodes in a gas. RF discharges are achieved either by capacitively or inductively coupling energy from a power supply into a plasma. Microwave discharges can be produced by coupling a microwave energy source to a discharge chamber containing a gas.
Plasma discharges can be generated in a manner such that both the charged species constituting the plasma and the neutral species, which can be activated by the plasma, are in intimate contact with the material being processed. Alternatively, the plasma discharge can be generated remotely from the material being processed, so that relatively few of the charged species come into contact with the material being processed, while the neutral species can still contact it. Such a plasma discharge is commonly termed a remote or downstream plasma discharge. As an example, a remote plasma source can generate plasma outside of a process chamber, such as in a remote device mounted on the chamber, and deliver reactive species and byproducts of the plasma into the process chamber. Depending on its construction, positioning relative to the material being processed, and operating conditions (e.g., gas species, pressure, flow rate, and power coupled into the plasma), a plasma source can have characteristics of either or both of these two general types.
A remote plasma source can be constructed as a toroidal plasma source in which electrical current is induced around a closed loop path inside of a gas, thereby creating a plasma that is approximately toroidal in shape. Specifically, in a remote toroidal plasma source, a gas enters the plasma source and is split into two paths that converge at the exit of the device to create a looped path of the gas inside of the plasma source. Electrical current is inductively coupled into the gas and circulate around the looped path to form toroidal-shaped plasma in the device. Additional gas enters the plasma source, mixes and reacts with the plasma in the plasma source, and then exits the plasma source carrying the reactive byproducts of the plasma to a process chamber.
There are currently two methods for constructing remote toroidal plasma sources. One involves constructing the internal plasma chamber of a remote toroidal plasma source entirely from a dielectric material, such as the methods disclosed in U.S. Pat. No. 7,501,600. The other approach involves constructing a plasma chamber by assembling metallic blocks (e.g., aluminum blocks) with internal cavities that are anodized to create a dielectric barrier to the gas and with dielectric breaks around the toroidal loop, such as the methods disclosed in U.S. Pat. Nos. 6,150,628 and 6,388,226. The second method of constructing a plasma source using metallic blocks is generally preferable due to the ease of fabrication and cooling and lowered cost.
In general, plasma created in a remote toroidal plasma source seeks a round shape along the inner diameter of the toroidal gas path because this is the shortest and lowest resistance path for current to circulate. However, in a toroidal plasma source made from one or more metallic blocks, a rounded shape is difficult to machine inside of the blocks. Instead, existing approaches machine relatively linear cavities into the metallic blocks and the resulting looped path is formed from multiple straight sections. For instance, FIG. 1 shows a prior art remote toroidal plasma source 100 with a plasma channel crated from two metallic blocks 102a, b with four straight channel segments 104a-d machined into the blocks 102a, b. The blocks 102a, b are assembled such that the channel segments 104a-d are positioned at right angles relative to each other. Thus, the resulting looped plasma channel 106 is rectangular with four nearly 90° turns 108a-d. One shortcoming associated with such an angular looped plasma channel 106 is that the plasma is likely to hit the inner walls of the inside corners of the turns 108a-d, which can erode the anodized coating in the channel, leading to poor performance, particle generation and premature device failure. In fact, dielectric failure from plasma-channel wall interactions constitutes a major failure mode and is a life-limiting event for a plasma source.
Substantial efforts and expenses have been put into preventing plasma interactions with inner walls of a plasma channel with limited success. One typical approach is to enlarge and control the radii of the internal corners of a plasma channel. However, access to these locations is limited due to difficulty in accessing the embedded angular turns, such as the 90° internal corners 108a-d of the plasma channel 106 of FIG. 1. Expensive flow based methods, such as vapor honing or hand polishing, are often used to obtain access to and enlarge these locations, which do not offer the cost or repeatability advantages of a fully automated machining process. Another known technique for limiting plasma-wall interactions is carefully tuned fluid mechanics. Generally, gas outside of a plasma has a much higher density than the plasma itself and this outside layer of gas can push the plasma away from the walls in the corner of a plasma channel and elsewhere. Thus, much effort has been focused on injecting the gas into a plasma channel in such a way as to create a protective layer that spins around the plasma in the plasm channel, thereby preventing the plasma from interacting with the channel walls. These efforts are often complex, expensive and involve additional structures to inject gas at multiple locations. For example, FIG. 2 shows another prior art remote toroidal plasma source 200 with a plasma channel 202 and one or more upstream gas plenums 204 in addition to the main gas inlet 206. These additional gas plenums 204 distribute gas injection locations to the sharp 90° corners 208 of the plasma channel 202, as described in U.S. Pat. No. 9,275,839. The additional injection points 204 used to spin the gas around the plasma in the plasma channel 202 are burdensome and expensive because they involve the construction of additional seals, coatings and parts with complex drill patterns.